1. Field of the Invention
The present invention relates to mass spectrometry and, more particularly, to an ion source that is positioned remotely from the spectrometer analytical cell.
2. Description of the Prior Art
Ion cyclotron resonance (ICR) is a known technique that has been usefully employed in the context of mass spectrometry. Typically, this technique has involved the formation of ions and their confinement and analysis within an analyzer cell. During analysis, the ions confined within the cell are excited and detected for spectral evaluation. In typical prior art systems, ion formation, trapping (confinement), excitation and detection all occur within the analyzer cell. An example of such a device is disclosed in U.S. Pat. No. 3,742,212, issued June 26, 1973, which is hereby incorporated by reference.
A later development, through which rapid and accurate mass spectroscopy became possible, employs Fourier Transform techniques and is commonly designated as Fourier Transform Mass Spectrometry (FTMS). This technique is disclosed in U.S. Pat. No. 3,937,955, issued Feb. 10, 1976, which is commonly owned with the present invention and which is also hereby incorporated by reference.
In conventional systems of the type described above, high resolution requires high magnetic field strengths and low operating pressures. To establish this environment, high field superconducting magnets and high speed vacuum pumping systems have been employed. As is known in the art, ions within this environment undergo a circular (orbital) motion known as cyclotron motion. This motion results from the thermal energy of the ions and the applied magnetic fields and is restricted in directions orthogonal to the magnetic field. It is conventional in the art to refer to directions orthogonal to the magnetic field in terms of X and Y axes which are axes orthogonal to the axis parallel to the magnetic flux lines--the parallel axis being commonly referred to as the Z axis.
During mass analysis, ions are restrained along the Z axis by electrostatic potentials applied to trapping plates. The mass analysis is performed either by measurement of the energy of an applied radio frequency excitation that is absorbed by the trapped ions at their cyclotron resonance frequency or by direct detection of the cyclotron frequency of the excited ions. Typically, the trapping plates are combined with other structures for ion excitation and detection to form an analyzer cell, the cell being positioned at the magnetic center of the superconducting magnet. At this magnetic center, and in the regions immediately adjacent, the magnetic field is generally homogeneous.
In conventional systems, it has been the practice to form ions for mass analysis within the analyzer cell. Ion forming techniques that have been employed include electron impact, laser desorption, cesium ion desorption, etc. In such systems, the transport of a sample to be analyzed to the analyzer cell for ionization (and analysis) has posed significant problems. These transport problems are compounded by the geometry of suitable superconducting magnets. In addition, sample introduction for ionization and analysis places significant demands on the high speed pumping systems that have been employed. Collisional damping of the ion signal, resulting from sample ionization and analysis in the same cell, reduces the mass resolution and sensitivity of the instrument. Magnet geometry also restricts placement of the ion formation devices and access to them.
As is apparent from the above, sample handling, including constraints imposed by system geometry, has limited the application of the described prior art ICR mass spectrometer systems.
One solution to the problem of increasing pressures resulting from sample introduction and ionization is disclosed in U.S. application Ser. No. 610,502 filed May 15, 1984 for Mass Spectrometer and Method, now U.S. Pat. No. 4,581,533 which is commonly owned with the present invention and which is hereby incorporated by reference. This system employs a cell of multiple sections and differential pumping. Sample introduction and ionization occurs in one cell section and analysis is performed in one or more other sections. Ion migration is permitted through the use of a conductance limit which allows the maintenance of a pressure differential between the cell sections and, accordingly, a differential pumping of those cell sections. The differential pumping allows an analyzer cell section at high vacuum. The separation of ion formation and analysis into distinct sections reduces collisional damping. However, the sample cell remains within the bore of the magnet. Thus, while sample handling problems are alleviated by this system, they are not fully addressed.
An alternative to the multiple-section cell, discussed above, is disclosed in U.S. Pat. No. 4,535,235 issued Aug. 13, 1985. In this system, a remote ion source is employed with a multiple stage rf quadrapole mass filter being employed to "transport" ions from the ion source to the analyzer cell. Differential pumping of the ion source and analysis section is provided. The ion source, being remote, allows easy access. Thus, sample handling difficulties associated with a common ion formation/analysis cells are ameliorated. However, the quadrapole arrangement is complex and contributes significantly to the system's size and cost. In addition, electrical interference from the quadrapole arrangement can affect the detection circuitry of the analyzer cell.